1. Introduction — What Commutation Actually Does

A brushless DC (BLDC) motor has no mechanical commutator, so the electronics must reproduce the brush timing: energize the right phase pair at the right electrical angle, then switch to the next pair as the rotor advances. That switching sequence is commutation, and its quality — the timing accuracy, the resolution (six steps vs continuous), the dead time, and the failure behavior — determines the torque ripple, the efficiency, the acoustic noise and the low-speed stability of the drive. This guide covers the motor and rotor-angle relationship, six-step (trapezoidal) commutation with Hall sensors, sensorless back-EMF (BEMF) zero-crossing detection, the dead-time and shoot-through problem, the comparison with sinusoidal FOC, the startup strategies, and a complete worked example of a 24 V, 8-pole, 100 W drive. It complements the BLDC FOC and H-bridge motor driver guides on this site.

2. Electrical vs Mechanical Angle and the Commutation Table

The commutation sequence repeats every electrical revolution, and the number of electrical cycles per mechanical revolution is half the pole count: θ_electrical = (P/2) · θ_mechanical, with P the number of poles. For an 8-pole motor (4 pole pairs), one mechanical turn is 4 electrical cycles. Six-step commutation has six states per electrical revolution, each holding one phase high, one low and one floating (30° of zero torque-free “idle” per state if driven with 120° blocks). The classic state table for phases A, B, C with a Hall sensor set is:

Step	High side	Low side	Floating	Hall state (H1H2H3)
1	A	B	C	101
2	A	C	B	100
3	B	C	A	110
4	B	A	C	010
5	C	A	B	011
6	C	B	A	001

Each Hall transition advances the state by one step, so the Hall edges are the commutation clock and the electrical speed is ω_e = 6 · T_step times the step rate. Hall placement is nominally 120° electrical apart (or 60° for some sensors), and a misaligned or reversed sensor causes a shifted commutation that raises current and heat for a given torque.

3. Six-Step with Hall Sensors

The simplest and most robust commutation is to read the three Hall inputs, look up the next state, and drive the three half-bridges; an EXTI or a polled ISR on the Hall lines advances the state at each edge. Key design points: (a) dead time between turning off one half-bridge and turning on the complementary one to prevent shoot-through; (b) the floating phase is used to sense BEMF if sensorless operation is also wanted; (c) the PWM can be applied on the high side, low side, or both (synchronous), and each choice affects conduction losses and the ability to sense BEMF. The Hall-based six-step is the workhorse for fans, pumps, drones and power tools because it is cheap, deterministic and needs no model — its weakness is the torque ripple at each of the six steps, which excites acoustic noise and limits low-speed smoothness.

4. Sensorless BEMF Zero-Crossing

Without Hall sensors the rotor angle is inferred from the back-EMF of the floating phase: the motor’s own rotation generates a voltage on the un-driven winding whose zero crossing (relative to the virtual neutral) occurs at a known offset from the ideal commutation instant. The BEMF magnitude is proportional to speed (E = K_e · ω), so sensorless detection fails at standstill and low speed — the drive must start open-loop. Practical details: (a) the divider and filter on the phase voltage create a phase lag that must be compensated by advancing the commutation angle with speed (typically 0–30° electrical); (b) the neutral point can be a real star connection or a derived virtual neutral from three matched resistors; (c) during the PWM off-time the phase is not driven and the BEMF is visible, so the sensing must be synchronized to the PWM (sample in the off window, or use a dedicated comparator with blanking). The reward is a Hall-less drive with lower cost and no sensor wires; the price is a startup ramp and a lower speed floor. The commutation timing and the PWM frequency itself can be checked with the timer/PWM calculator, which returns the timer prescaler and period for the carrier and the resulting step rate, and the motor’s operating current/torque/power point can be checked with the motor control calculator.

5. Dead Time, Shoot-Through and Gate Drive

Every half-bridge has a moment when both switches could conduct: if the upper and lower MOSFET are on simultaneously, the DC bus is shorted through them — a shoot-through that destroys the bridge. Dead time is the deliberate delay between turning one off and the other on, sized to be longer than the turn-off time of the switch including its gate-drive discharge: t_dead > t_off,max + margin, typically 200–500 ns for standard gate drives and 30–100 ns for fast GaN with strong drivers. Too little dead time → shoot-through risk; too much dead time → the body diode (or an external diode) conducts during the gap, raising losses and distorting the voltage. The dead time is inserted by the MCU’s complementary PWM with dead-time generator, or by the gate driver’s internal dead-time logic; verify with a scope across the half-bridge that there is no overlap across temperature and the gate-drive supply range. During the dead time the motor current freewheels through the low-side body diode (or, in synchronous rectification, the MOSFET channel after the dead time), and that freewheel path is the source of the voltage distortion compensated by the dead-time compensation in FOC.

6. Six-Step vs Sinusoidal FOC

Aspect	Six-step (trapezoidal)	Sinusoidal FOC
Angle resolution	6 steps / electrical rev	Continuous
Torque ripple	High (6th-harmonic)	Low
Acoustic noise	Higher	Low
Algorithm cost	Very low	High (Clarke/Park/PI/SVPWM)
Sensor	Hall or sensorless BEMF (coarse)	Hall + encoder, or observer
Efficiency	Good at high speed	Best over the whole range
Typical use	Fans, pumps, tools, drones	Servos, EVs, appliances, gimbals

FOC needs the rotor angle continuously to orient the current vector, so it depends on an accurate position source: an encoder/resolver for precision, or a state observer (sliding-mode, Luenberger, or a BEMF/flux observer) for sensorless. Six-step needs only the discrete Hall state or the BEMF zero crossing. The choice is a budget and performance decision: if the product tolerates the 6th-harmonic torque ripple and the acoustic signature, six-step is far cheaper in engineering and silicon; if smoothness, low noise or wide-range precision matters, FOC is the answer. Many drives run six-step for the startup and hand over to FOC once the rotor is spinning — a pragmatic combination that uses each where it is strongest.

7. Startup, Direction and Fault Handling

Sensorless drives need a startup sequence: (a) align the rotor by energizing a known phase pair briefly (forced alignment), then (b) ramp the commutation frequency open-loop while the current is limited, and (c) hand over to BEMF closed-loop once the BEMF is detectable. If the load is heavy or the inertia large, the ramp must be slow enough to keep the rotor synchronized. Direction is set by the order of the commutation table (reverse the sequence for reverse rotation). Fault handling must include: over-current (cycle-by-cycle or a hardware comparator), bus over/under-voltage, motor stall detection (no Hall transition or no BEMF zero-cross within a timeout), over-temperature, and the safe state on fault — all six low-side or all six off, depending on whether you want braking or freewheel. A stall at full current heats the winding fast, so the stall timeout must be short and the current limit must be set from the winding’s thermal limit, not from the MOSFET’s rating. For a driver-level view of the bridge and its protection, the H-bridge motor driver guide on this site covers the gate-drive and protection details.

8. Worked Example — 24 V, 8-Pole, 100 W Sensorless Drive

Target: a 24 V BLDC, 8 poles (4 pole pairs), 100 W at 3000 rpm, sensorless six-step with BEMF zero-crossing, smooth startup, dead time verified.

• Electrical speed: 3000 rpm × 4 pole pairs / 60 = 200 electrical revolutions per second → electrical frequency 200 Hz → step rate 6 × 200 = 1200 steps/s. Max Hall/step edge rate 1200 Hz (or the BEMF edge rate), which sets the ISR load.
• PWM: choose a 20 kHz carrier (above audible) → 100 PWM periods per electrical step at 3000 rpm; use the timer calculator to set the prescaler and period for a 20 kHz center-aligned PWM on the three complementary channels with the dead-time generator.
• Dead time: the MOSFET turn-off (plus gate discharge) measured 150 ns worst case → set 300 ns, and verify on the scope across the gate-drive supply range that the half-bridge never overlaps.
• BEMF sensing: the virtual neutral from three matched 10 kΩ resistors; the phase divider and RC filter give a lag compensated by advancing the commutation angle from 5° at low speed to 25° at 3000 rpm (mapped from speed).
• Startup: forced alignment 200 ms at 20% current, ramp the step rate to 300 steps/s over 1 s with a current limit, then hand over to zero-crossing detection; if no zero-cross within 50 ms, restart the ramp.
• Torque/current point: 100 W at 24 V ≈ 4.2 A average; the motor control calculator checks the torque constant, current and thermal margin, and confirms the 6th-harmonic torque ripple expected from six-step.
• Faults: over-current at 6 A (hardware comparator → bridge off), stall timeout 50 ms, bus OV/UV, and a safe-off state on any fault.

9. Common Mistakes

• Dead time too short or unchecked: shoot-through destroys the bridge on the first hard load; verify on the scope, not on the datasheet.
• Ignoring BEMF filter phase lag: the commutation angle is late, torque and efficiency fall, and the drive is noisy; advance the angle with speed.
• Startup without current limit: the forced-alignment and ramp phases can over-current the winding; always limit the current during startup.
• Mis-wired Hall order: a shifted or reversed sensor set gives a valid-looking but wrong commutation, with high current for the torque; verify the table against the physical sensor order.
• Stall without a timeout: a blocked rotor at full current heats the winding to failure within seconds.
• Choosing six-step for a servo application: the 6th-harmonic ripple and acoustic signature are unacceptable where FOC is needed; match the algorithm to the product.

10. FAQ

Q: How much dead time do I need? A: Longer than the switch’s worst-case turn-off plus margin — 200–500 ns for standard drives, 30–100 ns for fast GaN with strong drivers — and confirmed by scope at temperature extremes.

Q: Why does my sensorless drive fail at low speed? A: The BEMF is proportional to speed and vanishes near standstill; below the detection floor you must run open-loop (alignment + ramp) and hand over once the BEMF is detectable.

Q: Can six-step and FOC share a bridge? A: Yes — many drives start in six-step and switch to FOC; the bridge, gate driver and current sense are common, only the commutation algorithm changes.

Q: What causes the 6th-harmonic torque ripple? A: The discrete six-step current blocks interact with the motor’s BEMF shape, producing a torque ripple at six times the electrical frequency; sinusoidal FOC removes it.

11. Conclusion

Commutation is the timing core of a brushless drive: know the electrical angle from the pole count, drive the correct phase pair, respect the dead time, and sense the rotor angle either from Hall sensors or from BEMF with the phase lag compensated. Six-step is cheap and robust; FOC is smooth and precise; and the startup, direction and fault handling are what make either one survive in the field. Get the commutation right and the motor delivers its torque quietly, efficiently and for a long time.